A study in the US showed the drug had no benefit in treating coronavirus patients and was also associated with a higher risk of death.

The Food and Drug Administration has warned hydroxychloroquine can cause heart rhythm problems and Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said there was no evidence the drug could prevent coronavirus.

There have also been concerns raised in the US that the millions of people there who rely on it for treatment of lupus and rheumatoid arthritis could face more shortages because of an inevitable surge in demand for the drugs.

The UK government has said chloroquine and hydroxychloroquine are not licensed to treat or prevent COVID-19 and that the drugs should not be used outside clinical trials.

Prof Martin Llewelyn of Brighton and Sussex School, lead UK investigator for the MORU trial, said: "If drugs as well tolerated as chloroquine and hydroxychloroquine could reduce the chances of catching COVID-19 this would be incredibly valuable."

He added: "Like all drugs, chloroquine and hydroxychloroquine have side effects. But actually, used in the sort of doses that we will use them in the COPCOV trial, they're expected to be really, very safe."

Any adult who works in a UK healthcare facility and is delivering direct care to patients with proven or suspected COVID-19 can participate, as long as they have not been diagnosed with COVID-19 or an acute respiratory infection.